1. Field of the Invention
The present invention relates to a scanning type image forming apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus has been known, which has a MEMS (Micro Electro Mechanical System) mirror operating at a high speed while irradiated with laser light. In order to perform drawing with this image forming apparatus, a MEMS mirror may be resonantly driven to scan in the vertical direction and the horizontal direction at respective resonance frequencies, as drawing Lissajous figures (hereinafter “Lissajous scanning”). In this case, drawing points can be separated into a vertical scanning line Y representing a vertical component and a horizontal scanning line X representing a horizontal component. The vertical scanning line Y is represented by the following equation 1, where the angular frequency in vertical direction (resonance frequency) is ωy and time is t. Meanwhile, the horizontal scanning line X is represented by the following equation 2, where the angular frequency in vertical direction (resonance frequency) is ωx and time is t.Y=sin ωyt  (Equation 1)X=sin(ωxt+φ)  (Equation 2)
Here, φ represents the phase difference with respect to vertical scanning line Y. When there is no phase difference φ (φ=0), a drawing trajectory (scanning trajectory) in which outward journey (solid line) Ta and return journey (dashed line) Tb is evenly arranged, as shown in FIG. 6A. Here, FIG. 6A shows an exemplary Lissajous scanning to perform drawing for one frame within a period of time for a round trip (one cycle) in the vertical direction.
However, because the resonance frequencies of a MEMS mirror are changed due to a condition such as temperature, there is a problem that the phase difference φ between the vertical scanning line X and the horizontal scanning line Y changes to narrow the distance between the outward journey Ta (solid line) and the return journey Tb (dashed line) of the drawing trajectory, so that the density of the drawing trajectory may be changed (that is, the drawing trajectory is not even) depending on the drawing points.
To address the problem, a configuration as disclosed, for example, in Patent Literature 1 has been considered where a scanning line detecting means and a control means are provided. The scanning line detecting means can detect the distance between at least two adjacent scanning lines in the sub-scanning direction (vertical direction) and in the main scanning direction (horizontal direction). The control means controls the phase of the deflection angle of a deflector (MEMS mirror) such that the distance between the at least two adjacent scanning lines detected by the scanning line detecting means is fixed in the sub-scanning direction and the directions of the two scanning lines are opposite to one another in the main scanning direction.    Patent Literature 1: Japanese Patent Application Laid-Open No. 2008-216299 (claim 5 and paragraph 0021)
With this configuration, it is possible to ensure that the distance between adjacent scanning lines is fixed and Lissajous scanning is controlled precisely, and therefore improve the quality of an image formed on a scanning surface. Here, in order to realize a deflector such as a MEMS mirror with high operationality and low power consumption, it is necessary to drive the deflector at the resonance frequencies (natural frequencies). However, with the configuration disclosed in Patent literature 1, the respective driving frequencies of the deflector in the main scanning direction and the sub-scanning direction are controlled to vary from the proper resonance frequencies, and therefore the power consumption required to drive the deflector is likely to increase. Moreover, when the drive frequencies are significantly different from the resonance frequencies, the driving amplitude (deflection angle) of a deflector may decrease, and, if so, drive control itself is likely to be difficult.